Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are used as a solvent for an electrolyte salt for lithium secondary battery, solar cell, radical battery and capacitor. However since a flash point is low and combustibility is high, there is a danger of firing and explosion attributable to over-charging and over-heating, and in addition, since viscosity is high and conductivity at low temperatures is low, there is a problem that an output is decreased.
In order to solve those problems, it is proposed to use a fluorine-containing chain carbonate obtained by fluorinating a chain carbonate. For example, there are proposed carbonates in which an end of a fluorine-containing alkyl group is perfluoro, such as a carbonate having a fluorine-containing alkyl group at one end thereof and a hydrocarbon alkyl group at another end (JP6-219992A), a dicarbonate having an ethylene unit in its center and CF3(CF2)(CH2)— at both ends thereof (JP10-149840A) and a carbonate having hexafluoroisopropyl groups at both ends thereof (JP2001-256983A), and in addition, a carbonate having a fluorine-containing ether group terminated with a perfluoro group at one end thereof and a hydrocarbon alkyl group at another end thereof (JP2000-327634A).
However, carbonates having a fluorine-containing alkyl group terminated with perfluoro (JP10-149840A and JP2001-256983A) have disadvantages that solubility of an electrolyte salt is low and compatibility with hydrocarbon solvents is inferior. With respect to carbonates having a hydrocarbon alkyl group at one end thereof (JP6-219992A and JP2000-327634A), solubility of an electrolyte salt and compatibility with hydrocarbon solvents are improved, but there are lowering of flame retardance and decrease in a withstand voltage due to a decreased fluorine content.
On the other hand, there is proposed a method of adding a fluorine-containing ether (JP6-176768A, JP8-37024A, JP11-307123A and JP2000-294281A).
However, fluorine-containing ethers have lower viscosity at low temperatures as compared with carbonates having a fluorine-containing alkyl group at an end thereof, but there are disadvantages that solubility of a lithium salt as an electrolyte salt is low and compatibility with hydrocarbon carbonates used as a solvent for dissolving an electrolyte salt is low, thereby causing separation into two phases. In addition, since a flash point is relatively low, an improvement in flame retardance is necessary. Those are important problems to be solved.
Also in the case of electrolytic solutions for capacitors and radical batteries which repeat charging and discharging like lithium secondary batteries, improvement in flame retardance and a withstand voltage and enhancement of low temperature characteristics where viscosity is not high even at low temperatures and yet decrease in conductivity is small are desired.